1. Field of the Invention
This invention relates to digital PAL color television signal demodulators.
2. Description of the Prior Art
The PAL color television system uses suppressed carrier amplitude modulation by the U and V color components of quadrature sub-carriers. In a digital PAL color television system in which the analog composite video signal has been sampled, for example at the CCIR recommended frequency of 13.5 MHz, and the resulting samples have been coded prior to transmission or recording, it is necessary on reception or reproduction to regenerate the quadrature sub-carriers in order to demodulate the color components. Since the sampling is done at a regular frequency which is locked to the line frequency, it would in theory be possible to regenerate the quadrature sub-carrier waveforms by storing in a programmable read only memory (PROM) the phase values of the quadrature sub-carrier waveforms corresponding respectively to each sample position in the eight-field cycle over which the quadrature sub-carrier waveforms repeat. However, this would require a very large PROM because of the large number of phase values that would require to be stored.
The basic problem is to regenerate sub-carrier waveforms based on a line locked sample rate which, due to the frequencies used in the PAL color television system, has no easy mathematical relationship to the original subcarrier.